1. Field of Invention
This invention relates to local positioning systems, specifically to an improved local positioning system (LPS) for automated lawn mowers.
2. Discussion of Prior Art
Local positioning systems are known in the art. In my U.S. Pat. No. 5,974,347 (1999) I describe an automatic location system using three radio frequency (RF) transmitters to define circle equations which are solved simultaneously to determine the location of the mower. This approach requires accurate rotation control of a loop antenna and suffers from RF reflections that cause errors in measuring the angles used in the calculations. Also the transmitters had to have separate frequencies to identify them, and the figure-8 pattern of the directional antenna necessitated a sequencing circuit to verify sequences of the nulls received.
The problem of locating mobile platforms or robots simply and effectively has been attempted and partially solved by many different approaches. The techniques include dead reckoning, heading, ground-based RF beacons, and the GPS system. Time-of-flight (TOF) systems using audio, ultrasonic, and laser-based components have been used. Inertial navigation, active beam navigation systems, and optical positioning systems are used with triangulation and trilateration methods. Also landmark navigation with natural and artificial landmarks with visioning systems have been used.
The general problem of locating a mobile robot in its environment has not had a simple solution. Technical leaders in industry have been approached with the question of how to solve this problem cost effectively and were unable to come up with an answer.
U.S. Pat. No. 6,009,358 to Angott, et al. (1999) describes a programmable lawn mower using coded RF and audio signals. Coded signals are sent from the carriage or vehicle to each of the locating transmitters which send an acknowledgement, requiring a processor for codification and identification, adding to the complexity of the system. The signals from locating transmitters are continuous signals that may have interference from reflections from objects in the area causing errors and malfunctions. Further, the use of two microphones closely spaced on the vehicle to give angular position relative to the locating transmitter is not as accurate as utilizing a given greater distance between two locating transmitters.
U.S. Pat. No. 5,940,346 to Sadowski et al. (1996) describes a modular robotic platform with an acoustic navigation system. It requires three or more beacons, different RF frequencies per each beacon, a pseudo-noise generator, continuous wave acoustic signals, uses pulse modulation of the RF signal, requires advanced beacon design for accurate detection of the acoustic signal. It also suffers from Doppler frequency shift of the acoustic signal that must be taken into account.
A method of using acoustical signals to measure distance is given by Highfill in U.S. Pat. No. 6,590,834 (2003). This is applied to measuring the location of a rover carried by an individual in making surveying measurements in wooded areas. The method of determining distance uses an RF and an acoustic transmitter on the rover to transmit pulses to an array of receiver pods. Multiple pods are required to acquire data to be able to calculate the position of the rover.
U.S. Pat. No. 6,445,344 to Wise, et al. (2002), describes a local area positioning system utilizing multiple platforms in communication with each other in a local area network. It is similar to a global positioning system but on a more limited space providing three dimensional location of sensor platforms such as aircraft which would be an expensive, complex, and cumbersome system if applied to small moving vehicles such as an automated lawn mower.
U.S. Pat. No. 6,674,687 to Zeitzew (2004), uses a system and method for navigation using two-way ultrasonic positioning. However, the RF and ultrasonic signals are encoded and the two-way travel of the ultrasonic signal adds to the complexity of the system. The use of ultrasonic frequencies also has more attenuation in the air than lower audio frequencies.
U.S. Pat. No. 6,157,592 to Kriz, et al. (2000), discloses a three dimensional acoustic position determining system. It requires at least three transmitters, each coded for identification, and also requires a synchronization signal. The application is for determining the position of a receiver mounted on a person or object.
A method of measuring azimuth and distances from a moving platform and stationary reflectors is proposed by Noji, et al. in U.S. Pat. No. 5,011,288 (1991). The angle between adjacent reflectors is measured to calculate the azimuth. But the difficulty in measuring distances by phase differences in transmitted and reflected beams is that it requires a coherent light source something like a laser light emitting diode. Accurately measuring the phase difference is difficult, and also a minimum of three reflectors is required.
U.S. Pat. No. 6,995,708 to Schmidt (2006) describes a local positioning system to locate a cell phone user by sniffing earth based media and using bluetooth standards and GPS. Another system to locate a person with a cell phone is given in U.S. Pat. No. 6,748,224 to Chen, et al. (2004) describing a software implementation of a local positioning system It uses the radio propagation parameters in the CDMA and TDMA links of a cell phone system to establish the position of a digital cell phone.
U.S. Pat. No. 6,255,793 to Peless et al.(2001) describes a navigation method for autonomous machines using proximity sensors and markers defining a perimeter of the working area.
U.S. Pat. No. 5,956,250 to Gudat et al. (1999) describes a vehicle navigation system using VPS, GPS, and MPS. It includes accelerometers, gyros, odometers and Kalman filters, making a very complex and hence expensive system.
U.S. Pat. No. 5,758,298 to Guldner (1998) describes an autonomous navigation system for a mobile robot or manipulator and guides a robot to a predetermined point, for a transport or cleaning system. It uses a robot coordinate system for collision avoidance.
U.S. Pat. No. 5,495,427 to Puma et al. (1996) describes an ultrasonic position and orientation tracking system for monitoring articles in close proximity such as a pilot's helmet in a cockpit.
U.S. Pat. No. 6,393,360 to Ma (2002) is about a system for locating and directing a vehicle such as an automobile. It is a local positioning system assisting a driver in reaching their destination.
U.S. Pat. No. 5,955,973 to Anderson (1999) describes a field navigation system for navigating a farm vehicle in an agricultural field using a GPS system.
Local positioning apparatuses or systems described in U.S. Pat. No. 5,904,725 to Iisaka (1999), and 6,493,458 to Yasui, et al. (2002) relate to determining the position of a vehicle in traffic and traffic safety. These do not relate to locating an automatic lawn mower or roving platform.
U.S. Pat. No. 6,556,942 to Smith (2003) shows a short range indoor radiolocation system allowing the location of an item provided with an RF tag.
Other Publications:
Navigating Mobile Robots: Systems and Techniques by J. Borenstein, H. R. Everett, and L. Feng. Publisher: A. K. Peters, Ltd., Wellesley, Mass. c 1996. ISBN 156881058X oclc Number ocm34149992. This book surveys the state of the art in technologies used by a mobile robot to determine its position in the environment. Two groups are categorized: relative and absolute position measurements. Relative measurements include odometry and inertial navigation. Absolute measurements comprise active beacons, landmark recognition, and model matching. The most important result from this survey of mobile robot positioning is that there is no truly elegant solution for the problem.
Most sensors used for map building involve distance measurement. Three approaches are common: measuring time of flight (TOF) of a pulse of emitted energy traveling to a reflecting object, then echoing back to a receiver; phase-detection using continuous wave transmission; and frequency-modulated (FM) radar.
Potential sources of error for TOF systems include 1) variations in speed of propagation, 2) uncertainties in determining the exact time of arrival of the reflected pulse, 3) inaccuracies in timing circuitry, and 4) interaction of the incident wave with the target surface.
Detection uncertainties that were discussed relate to detecting a reflected pulse similar to a radar pulse.
While the inventors of the above LPS systems have made attempts to solve the problem of locating an automated lawn mower or moving platform, the prior art methods have failed to provide an elegant, simple, and satisfactory method required. There is a continuing need for improvements to such local positioning systems. In this respect, the present invention addresses that need.
Objects and Advantages
In all embodiments of this local positioning system (LPS), there is no use of reflected pulses hence this is not a problem. Also, variations in speed of propagation of sound caused by temperature differences can be taken into account and corrected by software. Circuitry inaccuracies are well below variations of the slow speed of sound in air so this is not a problem. Surface interaction does not apply because reflected energy is not utilized. It uses only a one-way travel of a sonic pulse that has the advantage of not being affected by specular reflection of an object's surface, and only the incident pulse is used for detection. Reflections from objects are not detected because they arrive later than the original incident pulse and thus have no effect. The disadvantages of the prior art are solved by my present LPS system, reducing the complexity, increasing reliability, and making it easier to manufacture.
Accordingly, several objects and advantages of some embodiments of the present LPS system are:                (a) To provide a simpler method of determining location employing fewer components and transmitting devices.        (b) To provide higher reliability by having fewer moving parts.        (c) To eliminate problems caused by specular audio reflections and RF reflections that interfere with depth of null measurements that use rotating loop antennas.        (d) To improve accuracy by measuring a one-way time of travel of an acoustic wave instead of an RF null.        (e) To eliminate the need for a sequencing circuit        (f) To reduce the amount of circuitry needed by eliminating the unique frequencies needed for RF triggering transmitters.        (g) To reduce the amount of complex mathematics needed to calculate coordinates by using simple trigonometric formulas instead of simultaneous solutions of circle equations.        (h) To overcome the time limitation for digitally pulsing the RF transmitter to trigger the locating sonic transmitters by frequency shift keying (FSK) the RF signal between two different analog modulating frequencies.        (i) To trigger the first locating transmitter on the leading edge of the FSK signal, and the second locating transmitter on the lagging edge.        (j) To shield and isolate the omni-directional sonic sensor from ambient noise as well as vibrations from the mechanics of a lawn mower.        (k) To eliminate coding of signals requiring identification by a processor.        (l) To eliminate interference of audio reflections from objects in the area.        (m) To reduce the number of beacons or locating transmitters required and eliminate the effect of Doppler frequency shift.        (n) To simplify the system by keeping it a two-dimensional instead of a three-dimensional location of sensor platforms, and by utilizing only a one-way travel of sonic signals instead of two-way travel.        (o) To eliminate separate synchronization signals.        (p) To provide a self-contained system that does not rely on bluetooth standards, GPS, or radio propagation parameters used in cell phone systems.        (q) To obviate the use of proximity sensors and markers to define working area perimeters.        (r) To reduce the complexity by eliminating accelerometers, odometers, gyros, and GPS systems.        
Further objects and advantages are to provide digital information that can be used to easily calculate the location of a roving vehicle and in particular an automated lawn mower. The system must be rugged and easy to manufacture and inexpensive for the consumer. Still further objects and advantages will become apparent from the ensuing description and drawings.